1.Field of the Invention.
The present invention relates to MOS Intergrate Circuit devices and more specifically to a back bias generator for complementary metal oxide semiconductor (CMOS) processes.
2. Prior Art
In the design of MOSFETs (metal-oxide semiconductor field- effect transistor), isolation considerations for high voltage circuitry often require the use of a back bias generator, especially on a device utilizing present CMOS processes. Various methods are known in the prior art for generating and regulating back bias voltages, such techniques being disclosed in U.S. Pat. No. 4,142,114; U.K. Patent GB No. 2,151,823; and European Patent EP No. 173,980. However, with the advent of the textured poly EEPROM (electrically erasable programmable read only memory) technology, voltages in excess of 20 volts DC are encountered by the semiconductor device. Because the FETs threshold voltage is a function of the back bias voltage (VBB), it is desirous to provide a back bias voltage which is substantially insensitive to temperature and power supply voltage changes.
In analyzing MOSFET behavior, the worst case field threshold isolation requirements dictate the least negative voltage value that the back bias generator needs to supply under worst case temperature and power supply conditions. The most negative voltage value of the back bias voltage generator directly influences the junction break-down voltage of the transistor and, therefore, this most negative value of VBB is the worst case condition for the junction break-down voltage of the transistor. The tradeoff between field transistor threshold voltage and junction break-down voltage is further constrained, in that an increase in the field implant dosage, which is achieved in order to increase the field transistor threshold voltage, will lead to a lower junction break-down voltage, and vice versa.
Therefore, based on these explanations, it is appreciated that a VBB which is insensitive to temperature and power supply voltage variations will alleviate high voltage isolation problems.